Navigation system are widely used today. On a monitor screen of a navigation system, typically, the current position of the vehicle and a route on which the vehicle is running are superimposed on a map image of a region in which the vehicle is currently travelling. For such a navigation system, it is essential to determine the current position as accurately as possible under various different driving conditions.
With such a navigation system, an output signal produced by a gyroscope which is indicative of changes in the vehicle travel direction is used. Namely, the output signal of the gyroscope shows amounts of angle of the vehicle with respect to a predetermined axis of the gyroscope. Each change of the angle represents a change in the travel direction of the vehicle in relation to a previously determined absolute travel direction.
The direction change information from the gyroscope is used in conjunction with distance information expressing a distance that has been traveled by the vehicle relative to a preceding estimated position of the vehicle. Typically, distance information is obtained based on an output signal from a vehicle speed sensor. The direction change information and the distance information are used to perform a “dead reckoning” processing to obtain the estimated current position and travel direction of the vehicle.
Although the sensors (gyroscope and speed sensor) of the vehicle are able to obtain relative position values with respect to the previous position, it is not possible to obtain absolute position values. Furthermore, the measurement by the gyroscope is based on detecting values of angular velocity of rotation about the predetermined axis of the gyroscope. When the axis does not accurately correspond to the axis about which the vehicle actually rotates, error will be involved in detecting the change of direction. Further, the signal produced from the gyroscope may include an offset even when an angular velocity is zero and such an offset varies due to various factors. Because of the above reasons, errors may arise in determining an amount of change in the vehicle travel direction, and the accuracy of detecting the current position of the vehicle will thereby be impaired. The errors will be accumulated by time, i.e., the longer the time, the larger the errors.
Because of the inaccuracy involved in the dead reckoning processing based on the on-board sensors, i.e., the gyroscope and speed sensor described above, the detected position is periodically corrected by the position measurement data calculated based on GPS signals from GPS satellites. Typically, absolute positions and corresponding travel directions of the vehicle derived from the GPS signals can be obtained at every one second (1 Hz). Because of noises contained in the GPS signals, the position data obtained from the GPS signals also contain errors, for example, about 100 meter radius.
The corrected vehicle position and travel direction estimates derived in the foregoing are combined to obtain an estimate of the path (road segment) which has been traveled by the vehicle up to the current position. The estimated travel path is applied to map data in a map matching processing. The path is compared with data expressing a road map of a region in which the vehicle is currently travelling, thereby further increasing the accuracy of a finally estimated current position of the vehicle. In this manner, it is possible to accurately display the route and the current position of the vehicle on the route.
However, appropriate GPS signals may not always be available, for example, in a downtown area with many tall buildings where GPS signals are reflected and interfered by the buildings. In another example, when a vehicle is in a valley between tall mountain walls, the minimum number of visible GPS satellites may not be available. In such a situation where the GPS signals are lost or insufficient, the position errors will be accumulated because the navigation system has to rely solely on the dead reckoning process.
FIG. 1 is a schematic diagram showing an example of situation where GPS signals may become invalid because of the tall buildings in a downtown area of a city. Since the GPS navigation requires the minimum number of visible GPS satellites, sufficient GPS signals may not be available depending on particular locations in such a downtown area. Generally, since GPS satellites stay in the same locations in the sky, such problems as GPS signal loss will happen at the same locations in the city.
FIG. 2 shows such a situation when a vehicle is travelling a downtown area in which a GPS signal is lost at a location A and is recovered at a location B. Thus, during that period, the navigation system has to rely solely on the dead reckoning processing. Because the error will be accumulated during the dead reckoning processing as noted above, even when the vehicle moves on the road indicated by the arrows in FIG. 2, the position Pd of the vehicle estimated by the navigation system may on the trace indicated by the dotted line. Then, by the map matching processing, the navigation system interprets that the vehicle position Pm is on the road segment indicated by the dotted line which is the street next to the street on which the vehicle is actually running.
Thus, every time when the user drives along the route indicated by the arrows in FIG. 2 for commuting to his office, for example, the navigation system may indicate the current vehicle position on the wrong road. Therefore, there is a need of a map matching method and apparatus for a navigation system which is able to more accurately estimate a position of a vehicle when a GPS signal is lost or otherwise unavailable.